[5][6] The formation consists of claystone, mudstone, siltstone, shale, diatomite, and conglomerates, with considerable regional variation, and was deposited in a moderately deep marine environment at a depth of approximately 500–5,000 feet (150–1,520 m).

The lowest portion of the formation in the Purisima Hills contains tar, for it is here that the unit forms an impermeable cap on the underlying Monterey reservoir of the Lompoc Oil Field.

[12] The Sisquoc Formation was deposited in a middle bathyal environment, at a depth of between 150 and 1500 meters, between approximately 4 and 6 million years ago – the upper Miocene and lower Pliocene eras.

Within Santa Barbara County, the Sisquoc alone has produced 127 separate finds, which are catalogued by the University of California, Berkeley Museum of Paleontology.

[17] In addition to the abundant diatoms which make up the diatomite, fossils include vertebrates such as sea lions and walruses, bony and cartilaginous fishes, and birds.

[18] In the Casmalia field, west of Santa Maria, pebbly and high-porosity units contain a considerable amount of oil, while clay-rich and diatomaceous mudstones contain almost none.

[19] The diatomites of the Sisquoc, like those of the Monterey, can function both as oil-bearing units – where they have been capped with impermeable beds – or they may be oil-free, in which case, if they are near the surface and of sufficient purity, they can be mined as diatomaceous earth.